Particleboard, which includes chipboard and fiberboard, is made from a mat of particles formed in the manner, for instance, described in U.S. Pat. Nos. 4,308,227, 4,341,135, and 4,315,722 of Dec. 29, 1981, July 27, 1982, and Feb. 16, 1982, respectively. The mat formed by such an arrangement is prepressed so it has modest dimensional stability, but must be pressed again, normally while heating it and under considerable pressure to produce rigid particleboard. The mat-forming equipment operates continuously, so it is necessary to use a continuously operating press that lies in the production line immediately downstream of the mat-forming station and immediately upstream of the finishing and trimming station.
Such a press can be of the platen type as described in commonly owned patent application Ser. No. 411,109 filed Aug. 24, 1982 by K. Gerhard, having upper and lower vertically spaced press platens confronting each other and forming a straight horizontal path that can be well over 1O m long. Respective upper and lower belts have confronting parallel stretches lying between the platens and flanking the path. Thus the belts are driven to move the mass of particles to be pressed along the path from an upstream end to a downstream end thereof. Press frames traversed by the platens and belt stretches bear vertically on one of the platens, normally supporting the lower platen. Heavy-duty hydraulic actuators are engaged between each frame and the other platen, normally the upper platen. The press frame is constituted as a plurality of frames traversed by the platens and belt stretches and formed as a single sheet-steel element at and adjacent the downstream end and as a group of substantially identical such elements at and adjacent the upstream end. A plurality of substantially identical hydraulic actuators are engaged between each frame and the other platen. The number of actuators per frame increases from the downstream end toward the upstream end. Thus the pressure exerted against the other platen is substantially greater at the upstream end than at the downstream end. These actuators are upright simple hydraulic rams and are all pressurized at the same pressure.
It is also known to use a belt-type arrangement provided with a drum having a heated outer surface and rotatable about an axis and at least one endless belt radially juxtaposed with a portion of the outer drum surface. Means is provided for tensioning the belt and thereby urging it radially toward the drum-surface portion so that the belt and the drum-surface portion form an angularly extending pressing region. The web to be compressed is fed tangentially to one end of the pressing region and is withdrawn from the other end thereof. In addition the drum is rotated to displace the web and the belt angularly along the portion so that the web is compressed against and heated by the drum.
In a standard such belt-type press used for making resin-impregnated paper or textile or for otherwise finishing hard laminates the belt is formed of flexible steel so that suffucient tension can be employed to provide the considerable compression--at least 1O kp/cm.sup.2 --needed to make laminates of resin-impregnated paper, to plastic-coat textile webs, or to finish particleboard or plywood. The steel belt is bent around the drum so that the tension in it is effective radially inward on the workpiece also partially wrapped around the drum.
In order that the belt be flexible enough to conform to the drum it must have a thickness which at most is one five-hundredth the minimum radius of curvature that the belt must conform to, which dimension normally is one-thousandth the drum diameter. With a modulus of elasticity of 2.1.10.sup.6 kp/cm.sup.2 the bending tension is 2,100 kp/cm.sup.2. A high-quality steel belt can therefore be operated with an overall tension of between 3,000 kp/cm.sup.2 and 4,500 kp/cm.sup.2. Thus even with a relatively high tension of 4,100 kp/cm.sup.2 only 2,000 kp/cm.sup.2 is effective for pressing, since 2,100 kp/cm.sup.2 of the force serves to bend the steel belt. As a result the considerable tension that a steel belt can be subjected to is largely wasted just bending it around the drum.
Accordingly German patent document No. 2,856,646 has proposed replacing the steel belt with one formed as a matrix of an elastomer in which reinforcement is imbedded. The reinforcement is steel cable and the matrix is a butyl rubber vulcanized to it. A silicon-rubber layer is fixed to the outer surfaces to make this belt hard. Such a belt is much more flexible than the above-described all-steel belt. Nonetheless its service life is very short at the high tensions necessary so it has not met with widespread use.